In areas where epidemics have occurred after 2001, each local government made all efforts to control measles (see IASR, Vol. 25, No.1). The Japan Medical Association, the Japan Pediatric Society, and Japan Pediatric Association have made recommendation on measles control (see p. 62 of this issue) and on establishing the Children's Immunization Week, which was brought into operation during March 1-7, 2004, including Saturday and Sunday (see p. 63 of this issue).
National Epidemiological Surveillance of Infectious Diseases (NESID): In compliance with the amendment of the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the Infectious Diseases Control Law) on November 5, 2003, measles was reclassified from Category IV to Category V, although the system of reporting from the sentinel clinics and hospitals has been unchanged.
Measles cases reported by pediatric sentinel clinics since 1982 are illustrated in Fig. 1. Although cases were the largest in number in 2001 since 1994, being 11.20 per sentinel (33,812 cases), the subsequent measles control measure has succeeded and cases in 2003 decreased to as few as 2.72 per sentinel (8,286 cases), one-fourth of that in 2001 and the fewest during the past 20 years. Weekly incidence during 2001-2003 showed a unimodal seasonality with a peak during the 17-19th week.
Incidence by prefecture (Fig. 2) shows that there were 20.0 or more cases per pediatric sentinel clinic in eight prefectures in 2001, which decreased to 0 and one prefecture in 2002 and 2003, respectively; prefectures giving less than 2.5 cases per sentinel markedly increased from four in 2001 to 26 and 33 prefectures in 2002 and 2003, respectively. In Hokkaido (see p. 66 of this issue), Kochi (see IASR, Vol. 22, No. 11, 2001), and Okinawa (see p. 64 of this issue), where the prefectures concentrated all power on measles control, more than 20.0 cases per sentinel in 2001 decreased suddenly to less than 2.5 cases per sentinel in 2002. Reported adult measles cases per sentinel hospital were more than 4.0 in seven prefectures in 2001, while such were in only two and three prefectures in 2002 and 2003, respectively. In Tokyo and Kanagawa prefectures, reported adult cases per sentinel were over 4.0 during the three consecutive years.
The ages of measles cases reported by pediatric sentinel clinics in 2001 (Fig. 3) include 1 year in 23%, 0 year in 15%, and 2 years in 10%, 0-2 years altogether accounting for 47% of all reported cases. In 2003, 0 year accounted for 16%, but 1 year decreased to 19% and 2 years to 7.3%. There was no change in 3-9 years. The ratios of elder children were 11% for 10-14 years, 3.5% for 15-19 years, and 2.1% for over 20 years in 2001, which increased to 15%, 6.3%, and 3.7%, respectively, in 2003. In the post-1984 and post-1991 epidemic periods (Fig. 4), overall cases decreased, and the ratio of 1-4 years increased and that of older than 5 years decreased; after the 2001 epidemic, cases were on the decrease in 2002 and 2003, nevertheless the ratio of 1 and 2 years decreased and that of older than 5 years increased, a phenomenon never seen before. Changes in the age distribution of adult measles cases reported by sentinel hospitals are seen, particularly the ratio of 25-29 years increased during 1999-2003 (Fig. 3).
Isolation of measles viruses: Reports of isolation of measles viruses by prefectural and municipal public health institutes (PHIs) (reports as of January 26, 2004) to the Infectious Disease Surveillance Center (IDSC), the National Institute of Infectious Diseases (NIID), are on the increase, 117 in 2001, 62 in 2002, and 177 in 2003, despite the decrease in number of cases, and each prefecture is tended to carry out virus strain survey. The majority of measles viruses isolated in Japan in 2001 belongs to a genotype of D5 (type D3 in Okinawa prefecture), and type H1, the principal type of the isolates in China and Korea, was isolated only in Kawasaki City and Tokyo (see IASR, Vol. 22, No. 11, 2001), whereas in 2002 and 2003, type H1 was isolated all over Japan (see IASR, Vol. 24, Nos. 1, 8, and 10, 2003 and p. 67-71 of this issue).
National Epidemiological Surveillance of Vaccine-Preventable Diseases (NESVPD) (see p. 71 of this issue): In the 2000 survey (see IASR, Vol. 22, No. 11, 2001), the antibody-positives with the gelatin-particle agglutination (PA) test ('positive' means a titer higher than or equal to 1:16) were 52% of 1 year and 79% of 2 years, while in the 2002 survey (Fig. 5), such increased to 73% of 1 year and 90% of 2 years. The measles vaccination coverage increased from 45% to 78%. This increase and the ages of cases described above seem to indicate the success of the campaign started in 2001, "Let's receive measles vaccination immediately after the 1st birthday." Meanwhile, antibody-positives among 0-year infants decreased from 83% to 67 % of 0 to 5-months babies and from 32% to 14% of 6-11 months, suggesting early diminishing of the transplacental antibody. Among children older than 90 months (7.5 years), the maximum age for routine immunization, there were still susceptible individuals in about 5% (Fig. 5).
The results of the countermeasure against measles and future subjects: Since measles epidemics occurring in Japan before 2001 were characterized by small- to medium-sizes always occurring in some areas among mainly nonimmune 1-year children, such a movement of earliest possible vaccination of children over 1 year has been seen all over the country. As a result, the vaccine coverage of 1-year children and the antibody-positive rate has increased. Cases reported in 2003 were the fewest during the past 20 years, counting about one-fourth of those in 2001. The ratios of 1- and 2-year child cases decreased particularly. Measles cases of over 10 years, however, increased and no decreasing tendency was seen in adult measles. In addition, outbreaks among college students occurred (see p. 67 & 69 of this issue). From now on, the epidemic itself should be controlled by increasing vaccination coverage at 18 months to 95% (see p. 62 of this issue), and it is time to discuss introducing two doses of measles vaccine, taking into account increasing cases of elder children. If cases continue to decrease as expected, notification of all measles cases, adopted by Okinawa and Ishikawa prefectures, will become necessary (see p. 64 & 67 of this issue). It is important to continue and intensify from now on investigating vaccine coverage, antibody prevalence, and viral strains.